1. Field of the Invention
This invention relates to electrical devices.
2. Introduction to the Invention
Many electrical devices comprise two laminar electrodes and, sandwiched between them, a laminar electrical element which may be a conductor, e.g. a resistive element, as for example in a resistor or a varistor, or a non-conductor, as for example in a capacitor. Particularly useful devices of this type comprise a resistive element which exhibits PTC (positive temperature coefficient) behavior, in particular circuit protection devices which comprise a main portion which is a laminate of two laminar electrodes and, sandwiched between the electrodes, a laminar resistive element which exhibits PTC behavior. The PTC resistive element may be composed of conductive polymer (this term being used to denote a composition comprising a polymer and, dispersed therein, a particulate conductive filler) or a ceramic, e.g. a doped barium titanate. When a conductive polymer is used, such devices are generally prepared by stamping (or otherwise cutting) a plurality of the devices out of a laminate of a sheet of the conductive polymer between two metal foils. When a ceramic is used, such devices are usually prepared by applying liquid electrode material to the major surfaces of a preformed laminar resistive element, and solidifying the liquid electrode material.
Devices of this kind can sometimes be used without the addition of electrical leads, for example by installation between two separate resilient terminals which bear on opposite faces of the device. However, it is difficult to obtain satisfactory results in this way. There is a delicate balance, which must be retained throughout the life of the device, between the minimum pressure needed for good electrical contact and good physical retention, and the maximum pressure that can be tolerated by the device without adverse effect on its properties. This problem is particularly severe when the PTC element is composed of a conductive polymer, because the conductive polymer is liable to creep or otherwise deform under pressure, particularly at elevated temperatures, e.g. when tripped (i.e. when converted into a high resistance high temperature state by a fault).
U.S. Pat. No. 4,924,204 (Uchida), the disclosure of which is incorporated herein by reference, proposes, in view of this problem, to make a laminar PTC conductive polymer device in which there are two offset areas which contain the PTC element and one only of the electrodes (e.g. opposite end areas of a rectangular device), and to connect the device via resilient terminals which bear on the respective electrodes in the areas where there is no opposite electrode. The terminals and the device are placed in an insulating casing which enables the terminals to press resiliently against the device. U.S. Pat. No. 4,924,204 states that the PTC element will not be deformed substantially, even if softened by self-heating, because the resilient forces of the terminals are distributed, and that, should the PTC element be deformed and one of the electrodes bent out of shape, the bent electrode will not contact the other electrode.